I understand the easter egg, but i'm hung up on one part. Wanted to see if I could get some clarification on this one.

When you place all 4 trample steams down there is the possibility that zombies could break the trample steams down during the round. Do you need to have all four active to get the tower to turn blue or can you keep flinging zombies even if you only have 1 trample steam still active while the others got taken down by the zombies??